The laser diode has been used frequently as a light source in a variety of photographic applications. For example, in the field of laser printing, a laser beam is focused through a lens and then scanned onto a photosensitive medium, such as film. The diode is often controlled by digital data stored in a computer. Image quality may be improved by forming an image from pixels having varying gray tones to form a continuous tone image in film. The continuous tone radiographic laser printer is one application in which a laser diode is used to expose radiographic film to electronic images produced by medical imaging modalities, such as computed tomography, magnetic resonance imaging, digital subtraction angiography, ultrasonic imaging, and the like.
A problem with laser diodes is that the illuminative output of a semiconductor laser diode is not linear across its entire operating range. More specifically, as shown in FIG. 1 (which is a typical graph of light output of a laser diode as a function of current), the resulting curve has a lower level non-linear operating region joined by a knee region to a higher level operating region where light output varies linearly as a function of input signal value. The linear region is called the lasing region and the non-linear lower level region is known as the light emitting or spontaneous emission region. The curve includes a lower portion between 0 and the knee current I.sub.k where the resulting optical power output P.sub.d varies in a non-linear manner as current I.sub.d changes and a second higher portion between I.sub.k and I.sub.max where optical power output varies linearly with changes in current I.sub.d. Because the non-linear region is unsatisfactory for generating a continuous tone image in response to image input signal levels, it is desirable that the laser output range be limited to the linear operating region. It is thus common practice to operate a laser diode at or above a predetermined current level called a threshold current. Problems arise, however, from attempting to bring the laser diode to threshold current concurrent with powering up the electronics, because the current drive circuits for the laser diode are generally unstable until power stabilizes and electrical glitches or over current conditions for the laser diode can result. Other problems arise from the fact that over the lifetime of the laser, more current is required to reach the same threshold illuminative output power, so that a system which applies the same threshold current to the laser diode over the life of the laser diode will be inadequate.
The following U.S. Pat. Nos. disclose laser diode control circuits which have not been entirely successful in solving these problems. U.S. Pat. No. 4,618,958 issued Oct. 21, 1986, inventors Shibata et al.; U.S. Pat. No. 4,663,760 issued May 5, 1987, inventors Shimada et al.; U.S. Pat. No. 4,763,334 issued Aug. 9, 1988, inventors Shimada et al.; U.S. Pat. No. 4,853,934, issued Aug. 1, 1989, inventor Sakurai; U.S. Pat. No. 4,856,011, issued Aug. 8, 1989, inventor Shimada et al.; U.S. Pat. No. 4,907,236, issued Mar. 6, 1990, inventor Shimada; U.S. Pat. 4,009,385, issued Feb. 22, 1977, inventor Sell; U.S. Pat. No. 4,876,442, issued Oct. 24, 1989, inventor Fukushima; U.S. Pat. No. 4,611,352, issued Sep. 9, 1986, inventors Fujito et al.; U.S. Pat. No. 4,995,045, issued Feb. 19, 1991, inventors Burley et al.; U.S. Pat. No. 4,733,398, issued Mar. 22, 1988, inventors Shivagaki et al.; U.S. Pat. No. 4,796,266, issued Jan. 3, 1989, inventors Banwell et al.; U.S. Pat. No. 4,918,681, issued Apr. 17, 1990, inventor Ikeda; and U.S. Pat. No. 4,890,288, issued Dec. 26, 1989, inventors Inuyama et al.